Field of the Invention
The present invention relates to an image forming apparatus.
Discussion of the Background
In an image forming apparatus of electrophotography, a toner image formed on an image carrier is transferred to a recording sheet at a transfer unit, and the recording sheet is heated and pressurized at a fixing unit to fix the toner image on the recording sheet in order to perform image formation. At the transfer unit, a transfer voltage is applied to a transfer roller to transfer electrified toner from the image carrier to the recording sheet. At the fixing unit, when heated by a heating roller, the toner, which has been transferred to the recording sheet at the transfer unit, is melted and fixed on the recording sheet by a pressurizing roller.
There has been known an image forming apparatus in which a recording sheet having a first side on which an image is formed (printed) as described above is circulated inside of the apparatus. This enables image formation on the second side of the recording sheet as well, thus performing duplex printing of the recording sheet (see Japanese Unexamined Patent Application Publication No. 2004-317603). When the image forming apparatus recited in Japanese Unexamined Patent Application Publication No. 2004-317603 performs duplex printing, a fixing temperature for the first-side printing is controlled to have an optimum value to prevent the recording sheet, which has been printed on the first side, from curling. Also, there has been disclosed a technique to the effect that, based on gradation processing with respect to an image to be formed on a recording sheet and its gradation value, a fixing temperature that enables fixing on the recording sheet is set (see Japanese Unexamined Patent Application Publication No. 2012-242751).
The contents of Japanese Unexamined Patent Application Publication No. 2004-317603 and Japanese Unexamined Patent Application Publication No. 2012-242751 are incorporated herein by reference in their entirety.
Due to an influence of the transfer voltage, an electric discharge may occur at a transfer nip area in the transfer unit. Such an electric discharge in the transfer unit may cause electric discharge noise called white dots in an image formed on a recording sheet. These white dots on the image formed on the recording sheet are generated in a state in which an electric discharge is liable to occur, for example, when a sheet resistance of the recording sheet is large and when a toner electrification amount is high. In the case of duplex printing, heat for fixing in the first-side printing decreases the moisture of the recording sheet to increase the sheet resistance. Therefore, white dots are liable to be generated in the second-side printing. Moreover, in recent years, there has been a demand for increasing the conveyance speed, and consequently, the transfer voltage at the transfer unit is increased to cause white dots more likely.
In the image forming apparatus recited in Japanese Unexamined Patent Application Publication No. 2004-317603, the fixing temperature for the first-side printing is controlled to have an optimum value to prevent a conveyance failure at the time of the second-side printing due to curling of the recording sheet. However, the fixing temperature for the second-side printing is constant. Consequently, depending on the fixing temperature set for the first-side printing, an amount of heat applied to the recording sheet may be insufficient as compared with a heat amount required for duplex printing. This may unfortunately cause a fixing defect. In the image forming apparatus recited in Japanese Unexamined Patent Application Publication No. 2012-242751, the fixing temperature is controlled to have an optimum value when simplex printing is repeated. However, when the image forming apparatus is applied to duplex printing, the fixing temperature becomes high at the time of the first-side printing. This increases the resistance of the recording sheet in the second-side printing, which may cause electric discharge noise.
In view of these problems, it is an object of the present invention to provide an image forming apparatus that eliminates or minimizes electric discharge noise at the time of duplex printing and applies a sufficient amount of heat for fixing to a recording sheet.